When taking a photograph of a subject under backlit conditions, e.g. against the sun, the image of the subject portion is an indistinct backlit image with low lightness and contrast, making it difficult to make out details of the subject. Image processing is performed to improve the quality of these backlit images, as well as images suffering in quality due to overexposure, underexposure, blurring due to jiggling when capturing the image, noise, insufficient light, and the like, in order to improve lightness and contrast in the image. One method used in conventional image processing is a Retinex process.
The Retinex process preserves the input image data for high-quality regions and improves the image quality primarily in low-quality regions. The Retinex process performs convolution operation by using Gaussian filters to calibrate pixel data in the original image to values reflecting data of surrounding pixels, calculates reflectance component data for the original image from the natural logarithm of the calibrated pixel data, and calculates illuminance component data by dividing pixel data of the original image by reflectance component data of the original image. Specifically, this process divides the original image into reflectance components and illuminance components. Next, a process is performed to calibrate the brightness and level (contrast) of the illuminance component through gamma correction or the like and subsequently generates improved image data for the original image in backlit portions and the like by combining the calibrated illuminance components with the reflectance components.
U.S. Pat. No. 6,885,482 (corresponding to Japanese Patent Application Publication No. 2001-69525) discloses a method of converting RGB values to a color space configured of a luminance component and color components, such as YCbCr or YIQ, performing the Retinex process only on the luminance component Y, while maintaining the color components, and converting the values back to RGB. Calibrating only the luminance component (i.e. not calibrating the color components) prevents an upset in the color balance and a shift in color. Further, since the Retinex process is performed only on the luminance component, this method requires fewer calculations than a method for performing the Retinex process on each RGB plane and, hence, can complete the process quicker.
U.S. Patent Application Publication No. 2004/0091164 (corresponding to Japanese Patent No. 3,731,577) discloses a method for improving the processing speed in the Retinex process. This method forms a reduced image (low-resolution image) from the original image using the average pixel method or the like, forms a peripheral average pixel value image (blurred image) by finding the peripheral average pixel value for each pixel in the reduced image, forms a Retinex processed image from the original image and an image produced by expanding the blurred image, and forms an output image from the original image and the Retinex processed image.